It is estimated that, in a future decade, mobile broadband traffic will be more than one thousand times larger than the mobile broadband traffic of today. Bandwidth of a relatively low conventional frequency band cannot meet service requirements brought by this explosive growth. Therefore, using a millimeter-wave band (30 GHz to 300 GHz) with abundant bandwidth resources as a backhaul frequency and an access frequency will become a development trend in the industry.
Compared with communication at a relatively low frequency band, millimeter-wave communication is more easily affected by factors such as rain fade and atmospheric absorption, resulting in a large path loss. To ensure a specific propagation distance, a beamforming technology needs to be used for the millimeter-wave band, to obtain good link transmission quality.
Beamforming derives from a concept of an adaptive antenna and can be classified into beamforming at a transmit end and beamforming at a receive end. Beamforming at the transmit end means that amplitude and/or phase adjustment is performed on a feed of an antenna array element, to form a specific transmit signal. From a perspective of an antenna pattern, a beam in a specified direction is formed. Beamforming at the receive end means that a weighted synthesis is performed on various signals received by a plurality of antenna array elements, to form a directivity pattern of a required spatial shape. It should be noted that a prerequisite for using the beamforming technology is that a plurality of antennas should be deployed on at least one of a transmitter or a receiver on a communications link. FIG. 1 shows an example of beamforming at a transmit end, where a group of antennas (an antenna array) usually forms a directional beam.
However, a beam formed after beamforming is used for the millimeter-wave band is usually narrow. Therefore, in an access scenario, to ensure a specific coverage area, a typical solution is to use a beam training technology.
A basic principle of the beam training technology is to adjust, according to information such as a user location and a channel state, directions of directional beams generated by antennas of a transmit end and a receive end on a communications link, so that the beams of the transmit end and/or the receive end are spatially “aligned”, to obtain a relatively high antenna gain. A beam of a base station points to a user terminal as much as possible by means of beam training, as shown in FIG. 2a. If beam training is also used at the user terminal, beams of the base station and the user terminal are “aligned” as much as possible, as shown in FIG. 2b. 
The beam training technology has been commercially available in the IEEE 802.11ad standard, which is a commercial wireless local area network standard for a 60 GHz frequency band. A beam training method defined in the IEEE 802.11ad standard supports only a single input single output (SISO) transmission mode. That is, both a transmit end and a receive end use only a single beam to send and receive signals.
Obviously, the beam training method in the IEEE 802.11ad does not support a multiple input multiple output (MIMO) mode. To support the multiple input multiple output mode, a beam training solution in a MIMO mode is proposed in a current system based on the solution provided in the IEEE 802.11ad. In the MIMO mode, it is required only to select N groups of “aligned” directional beams according to a quantity of streams transmitted in parallel using MIMO, and generate antenna pairs corresponding to the N groups of directional beams. In addition, a single antenna (which is an array antenna) can generate only one beam direction in a single time segment. Therefore, specific implementation may be as follows.
When determining N transmit antenna beam directions and receive antenna beam directions that are finally selected, a station (STA) needs to select N transmit antenna beam directions from N different transmit antennas, that is, to form N antenna pairs that generate directional beams.
It can be learned from the beam training implementation solution in the MIMO mode that N directional beam pairs selected by a responder of beam training need to be generated by different transmit/receive antenna pairs. If the technical solution is used in a scenario of multiple responders (for example, STAs), a conflict may occur when the multiple responders select the transmit antennas of an initiator to generate a directional beam antenna.